The measurement of porosity, i.e., the quantification of the volume of the formation that is made up of pore space rather than solid rock matrix, is one of the key measurements used to quantify oil and gas reserves in the underground formation, or to quantify the underground volume capacity for various underground storage or disposal applications.
The pores in underground formations are typically filled with a mixture of water and hydrocarbon(s) molecules in the liquid state, but may comprise as well H2S, CO2, N2, etc. molecules and may consist more generally of a mixture of solid, liquid and gas phases in mechanical and thermodynamic equilibrium. Moreover, if solvents are present (such as water), then salts (such as NaCl or KCl/etc.) are usually also present as solute, and the corresponding solution ions may alter the various characteristics of the solvent, in significant ways.
Thus, one of the issues that traditionally affect the measurement of porosity is the kind of substances present inside in the pores of the formation. This is because there does not exist an ideal and standalone porosity measurement that would always read correct underground formation porosity, irrespective of the substances present inside the pore space. Hydrogen Index (HI) measurements for example, which are sensitive to the number of hydrogen atoms in the pore space, may provide a means to estimate underground formation porosity, when only water and oil are present in the pores. However, problems occur in the presence of a gas in the pores, because gas will have substantially fewer hydrogen atoms per unit volume, as compared to water and oil, which have roughly similar HI. Another example is that of Density (rho) measurements, which are sensitive to the density of the substances present inside the pore space, and may provide a means to estimate underground formation porosity, when only water and oil are present in the pores, provided the density of the rock matrix is also known. However, problems occur also in the presence of gas in the pores, because gas is significantly lighter than water and oil, which have roughly similar density. Consequently, an increase in gas-filled porosity will lead to both a decrease in Hydrogen Index and a decrease in Density, which would respectively indicate a reduced apparent porosity as derived from the Hydrogen Index measurement, and an increased apparent porosity as derived from the Density measurement; the so-called crossover effect. The chemical composition and the pressure and temperature of each gas, have a direct bearing for example on the average number of hydrogen atoms per molecule of gas, and on the gas density, which will also affect the apparent porosity measurements such as derived from the Hydrogen Index measurement and the Density measurement.
A further issue which affects correct porosity measurement is the invasion of the drilling mud filtrate into the underground formation. Specifically, in LWD (Logging While Drilling) applications this is most noticeable, since the LWD measurements are typically acquired a few minutes to a few hours only after underground formation have been freshly drilled. As such, the drilling mud filtrate will have invaded into the formation only up to a few inches or so. As time progresses, the drilling mud filtrate penetrates deeper into the formation, which may affect certain measurements and result in invasion-dependent or time-dependent measurement readings. Specifically, in the situation of an LWD tool relying on multiple measurements each having a different radial depth-of-investigation into the formation, then these measurements would be affected differently by the mud-filtrate invasion profile into the formation, especially when the radial depth-of-investigation of a first measurement is within the Invaded Zone, whereas the other measurement has a radial depth-of-investigation that extends beyond the Invaded Zone (i.e., into the Virgin Zone) and is thus typically less affected by invasion. Traditionally, most of the techniques dealing with the estimation of porosity in underground formations were developed for WL (Wireline) applications, prior to the advent of LWD logging. And because WL measurements are usually acquired days after the borehole has been drilled, the Invaded Zone typically extends a foot or more into the underground formation at that time, and invasion issues are not of as great concern as for LWD measurements.